1. Field of the Invention
The invention relates generally to navigation and more specifically to equipment and methods for communicating earth position data to and from a GPS receiver.
2. Description of the Prior Art
Automatic piloting of ships and boats has been traditionally based on magnetic compass or wind direction. Flux-gate compasses, for example, are widely used in auto-tillers for sailboats. A servo system is employed to keep the boat's heading fixed relative to the earth's magnetic poles. Mechanical rudders combined with wind vanes have long been used to steer sailboats to maintain a particular attack angle of the boat into the wind, or windpoint.
Recent developments in satellite-based navigation systems have allowed global positioning system (GPS) to be adapted to ship and small boat automatic piloting. Several manufacturers of such equipment in the United States have joined with one another to standardize the format of data output from GPS system equipment so that charting and piloting equipment can be mixed and matched with various GPS receivers.
The typical format offered commercially in the United States is promulgated by the National Electronic Marine Association (NEMA), and involves the communication of position expressed in latitude and longitude. However, the use of latitude and longitude implies the use of a datum which also must be communicated, or at least agreed upon. Typical datums in use in the United States are "WGS-84" for road maps and "AND-27" for topographical maps. In Japan, the old "Tokyo Datum" is in common usage. Unfortunately, datums are not readily, or commonly, automatically communicated between instruments.
Numerous datums are in common usage throughout the world. Therefore, portable hand-held GPS receivers are conventionally programmed to offer a variety of standard datums and even a few custom datums. For example, the SCOUT.TM., as marketed by Trimble Navigation, Ltd. (Sunnyvale, Calif.), has 123 datums and two custom datums in its memory that can be selected from a user menu which is presented on a display screen.
The upload and download communication of latitude and longitude information between a computer and a portable GPS receiver can therefore be undesirable and a better method and apparatus are needed for the accurate exchange of position information between navigation instruments.